Are there kinematic equations that are not always true?

• Lucho G
In summary: I agree. Every equation (Or at least most that I can think of) and their assumptions are a...package?
Lucho G
Homework Statement
Are there kinematic equations that are not always true? If so, which ones and in what cases?
Relevant Equations
Cinematica
please I need to clarify this question
thanks sincerely
Luis

Delta2
What do you mean by "true"? There are many equations which have a realm of validity, i.e. that rely on specific assumptions, if that's what you're asking.

PhDeezNutz, Delta2 and Lucho G
etotheipi said:
What do you mean by "true"? There are many equations which have a realm of validity, i.e. that rely on specific assumptions, if that's what you're asking.
i wanted to say the equations that are not always fulfilled in the kinematic.

Delta2
Lucho G said:
i wanted to say the equations that are not always fulfilled in the kinematic.
Any equation is only valid in specified circumstances. Even then, some are only approximations valid over a certain range.
You will need to be more specific about which equations concern you.

Delta2
Lucho G said:
Homework Statement:: Are there kinematic equations that are not always true? If so, which ones and in what cases?
Let's assume :
1) by 'kinematics equations' you mean the elementary equations of constant acceleration, e.g. https://www.onlinemathlearning.com/image-files/suvat-equations.png
2) by 'true' you really mean 'applicable' or 'accurate'.

The equations are typically used for motion in a straight line with constant acceleration.

The equations are not applicable if the acceleration is not constant. A simple example is an object fallng through the air when air resistance is present. Can you explain why the acceleration is not constant in this case? (Hint, air resistance increases as speed increases).

Delta2
Lucho G said:
Homework Statement:: Are there kinematic equations that are not always true? If so, which ones and in what cases?
Relevant Equations:: Cinematica

please I need to clarify this question
thanks sincerely
Luis
Here is one. Distance = Speed × Time is not true in the case when the object accelerates. Can you think of another one? You have already received some hints.

the most prominent i can think about is the "max range" projectile equations. all of them assume 45 degrees as the optimal angle for launch. however that may not be true if the launch level and the landing level are different. which means all the shortcut equations related to this special case "burn and die".

maxwells_demon said:
the most prominent i can think about is the "max range" projectile equations. all of them assume 45 degrees as the optimal angle for launch. however that may not be true if the launch level and the landing level are different. which means all the shortcut equations related to this special case "burn and die".
You get maximum range in projectile motion regardless of difference between launch and landing level if the initial velocity and the final velocity vectors are perpendicular. The 45o projection angle is a special case of that when the vertical velocity component just changes sign upon landing. You can find the details here.

All the kinematics equations you are likely to be exposed to at the freshmen and sophomore level assume constant acceleration.

Throw constant acceleration out the window and the formulas are no longer valid (Obviously).

PhDeezNutz said:
All the kinematics equations you are likely to be exposed to at the freshmen and sophomore level assume constant acceleration.

Throw constant acceleration out the window and the formulas are no longer valid (Obviously).
All? Counter-example: Simple harmonic motion.

PhDeezNutz
kuruman said:
You get maximum range in projectile motion regardless of difference between launch and landing level if the initial velocity and the final velocity vectors are perpendicular. The 45o projection angle is a special case of that when the vertical velocity component just changes sign upon landing. You can find the details here.

oh yeah. mb. a faulty inference.

well, for what it's worth, all kinematic equations assume negligible air drag, and frictionless surfaces. all of them go out the window when we account for just how objects act irl.

PhDeezNutz said:
All the kinematics equations you are likely to be exposed to at the freshmen and sophomore level assume constant acceleration.
Specifically, the SUVAT equations.
But I see it as wrong to say the equations are not always 'true'. As I posted, all equations come with specifications of what the variables mean and under what conditions they apply; it's a package. The SUVAT equations are always true, but they can be misused.
For SUVAT it's not just that acceleration is constant, you also need to ensure displacement, velocity and acceleration are all being measured in the same straight line.

PhDeezNutz
kuruman said:
All? Counter-example: Simple harmonic motion.

fair point.

haruspex said:
Specifically, the SUVAT equations.
But I see it as wrong to say the equations are not always 'true'. As I posted, all equations come with specifications of what the variables mean and under what conditions they apply; it's a package. The SUVAT equations are always true, but they can be misused.
For SUVAT it's not just that acceleration is constant, you also need to ensure displacement, velocity and acceleration are all being measured in the same straight line.

I agree. Every equation (Or at least most that I can think of) and their assumptions are a package deal.

1. What are kinematic equations?

Kinematic equations are mathematical equations that describe the motion of objects without considering the forces that cause the motion. They are commonly used in physics to analyze and predict the motion of objects.

2. Are all kinematic equations always true?

No, not all kinematic equations are always true. Some equations are only valid for certain types of motion, such as constant acceleration or constant velocity. It is important to use the correct equation for the specific situation being analyzed.

3. Which kinematic equations are not always true?

The equations for position, velocity, and acceleration are always true, but equations involving time, displacement, and acceleration due to gravity may not always be true depending on the specific conditions of the motion.

4. How can I determine if a kinematic equation is true for a specific situation?

To determine if a kinematic equation is true for a specific situation, you must first identify the type of motion being analyzed (e.g. constant acceleration or free fall) and then use the appropriate equation for that type of motion. It is also important to double check the units and variables used in the equation to ensure they match the given situation.

5. What happens if I use the wrong kinematic equation?

If you use the wrong kinematic equation, you will likely get an incorrect result for the motion being analyzed. This can lead to inaccurate predictions and conclusions about the motion. It is important to carefully select and use the correct equation to ensure accurate analysis and predictions.

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